There are a variety of situations in which it is desired to expand a tube radially to form a tight, leak-free joint. For example, large heat exchangers, particularly the type used as steam generators in nuclear power plants, often employ a tube sheet, which is a steel plate several feet thick, through which hundreds of stainless steel or carbon steel tubes must pass. The tube sheet is initially fabricated with bores of a suitable diameter in which the tubes are inserted. The tubes are then expanded radially against the sides of the bores by plastic deformation to permanently seal the small crevices that would otherwise exist around the tubes. If these crevices were allowed to remain, they could collect corrosive agents, and would, therefore, decrease the reliable and predictable life-expectancy of the equipment.
One known type of swaging apparatus employs a drawbar encircled by elastically deformable rings, which may be made of polyurethane. The drawbar is inserted axially into the structure to be expanded and is then retracted into a head, causing the pressurization rings to be compressed axially and expanded radially. Apparatus of this type may be used to perform the entire swaging operation, or it may advantageously be used to perform a preliminary step followed by hydraulic swaging, particularly in high pressure applications.
When a typical tubular structure expands under swaging pressure, the expansion does not end abruptly at the ends of the intended pressure zone defined by the outer ends of the outermost pressurizing rings. Instead, the structure in which the drawbar is inserted is expanded beyond the pressure zone, with the expansion tapering off gradually along a transitional portion to the unexpanded diameter. The application of swaging pressure therefore creates an annular void at each end of the intended pressure zone within the transitional portions of the swaged structure.
At extremely high pressures, the pressurization rings tend to behave as a liquid and deform to fill any available voids. Thus a ring adjacent a void will often be extruded into the void. The shape and depth of the voids created in a typical swaging situation is such that the elastic limits of the material are exceeded. The apparatus can be permanently damaged and it may be difficult to remove the apparatus from the expanded structure.
An objective of the present invention is to provide an improved drawbar swaging apparatus in which the problem of destructive inelastic extrusion of the pressurization ring or rings is minimized or eliminated.